Dr ME - Stoichiometry, acids and bases

Define stoichiometry

Calculations relating the amount of products and reactants in chemical reactions

Define theoretical yield

Maximum mass of product that can be obtained assuming all of the limiting reagent is consumed.

Define actual yield

Amount of product actually obtained in the experiment. Can only be determined by doing the experiment.

Equation to calculate percentage yield

Define percentage purity

In a given mass of material it is the percentage amount (mass) of the material of interest

What does the Valence shell electron pair repulsion theory suggest?

• Predicts the shape based on tetrahedral - 2 bonding pairs and 2 lone pairs of electrons

• Lone pairs are more sterically demanding (bigger) that bonding pairs, so the bonding pairs are pushed closer together.

What is the Arrhenius theory of acids and bases

• Acids release H+ ions into water

• Bases release OH- ions into water

Bronsted-Lowry definitions of acids and bases

Acids - H+ donors

Bases - H+ acceptors

Most acids and bases fit the Arrhenius equation except ammonia - it has basic properties but it isn’t the source of OH- ions.

Define amphoteric

Can act as either acid or base

Define amphioprotic

Can accept or donate H+

Define Lewis acids and bases

Acid - electron pair acceptor

Base - electron pair donor

Define ligand

Species that can donate an electron pair to the metal i.e. any lewis base

Define coordinate bond

A bond in which both the shared electrons come from one of the atoms.

Explain the 2 factors that affect acid strength

1. H-A Bond strength - If the bond is strong then the acid will be weak as energy is required to break the H-A bond.

2. If proton is bonded to a more electronegative atom the acid will be stronger.

   • Because in H - A if A is more electronegative there is a stronger pull on the shared electron.

   • There is a more polar bond

   • Easier to break the bond

What are binary acids?

Acids formed by halogens e.g. HF, HCl, HBr, HI

HI Has the weakest bond so dissociates more easily and is the strongest acid.

Bond strengths decrease from F to I and acid strength increases from F to I.

What are hydrides of 1st row elements?

CH4, NH3, H20, HF

Large increase in electronegativity from C to F leads to increasing polarity of the bond from C to F so easier bond dissociation.

Carboxylic acids

They have neighbouring polar C=O that increases the polarity of O-H. They are medium weak acids.

Alcohols

Contain a polar OH group but have no neighbouring C=O groups so they’re very weak acids.

Structure and name of HNO2

Nitrous

Structure and name of HNO3

Nitric

Name and structure of H2SO3

Sulforous

Name and structure of H2SO4

Sulphuric

Name and structure of H2CO3

Carbonic

Name and structure of HOCl

Hypochlorus (bleach)

Name and structure of HClO2

Chlorous

Name and structure of HClO3

Chloric

Name and structure of HClO4

Perchloric

Name and structure of H3PO4

Phosphoric

Name and structure of H3PO3

Phosphorus

Three factors affecting acidity

1. Oxidation number of central atom

   • Normally positive

   • If large and positive, it exerts a strong pull on electrons shared with oxygen.

2. Size of central atom

   • For central atoms with the same ox number, the e withdrawing effect is greater for the smaller ion as it has a higher charge density.

3. Electronegativity of central atom

   • A measure of how strongly it attracts the e.

What is an indicator?

It is a large water soluble weak acid or base that changes colour in some known pH range.

Methyl Orange

Red to yellow

3.2 - 4.2

Methyl Red

Pink to yellow

4.2 - 6.2

Bromothymol blue

Yellow to blue

6.0 - 7.6

Phenolphthalein

Colourless to red

8.0 - 10.0

Equivalence point

The point of chemical equivalence when the number of mols of acid = number of mols of base

End point

Point at which the titration has stopped

What is a primary standard?

A very pure compound that can be accurately weighed out to give a reliably known number of moles from which a solution can be made up. The concentration of this solution is then reliably known, so can be used to standardise other solutions.

Requirements of primary standards

1. High purity

2. Air stable

3. No hydrate water

4. Cheap and readily available

5. Adequate solubility

6. Reasonably large formula weight - reduces relative error on weighing

What is the solvent levelling effect?

Where a solvent limits the apparent strength of very strong acids or bases dissolved in it, making them appear equally strong by reacting completely with the solvent.